Knee joint prosthesis and related method

ABSTRACT

The present invention relates to a knee joint prosthesis having a tibial component and a femoral component. The tibial component includes a fixation portion fixed to an upper end of a prepared tibia and a ceramic bearing portion presenting articulation surface(s). The femoral component is fixed to a lower end of a prepared femur and comprises a ceramic body portion presenting articulation surface(s). The respective articulation surfaces of the tibial and femoral components are configured for essentially congruent engagement over a full range of movement of the prosthesis, and the bearing portion of the tibial component is adapted for movement relative to the fixation portion. Also disclosed is an apparatus and a method for finishing articulation surfaces of ceramic components of a knee joint prosthesis, including the steps of combining the ceramic bearing portion and the ceramic body portion, and imparting relative movement to the bearing portion and/or the body portion.

CROSS REFERENCE TO RELATED APPLICATIONS

This application represents the national stage application ofInternational Application PCT/EP2010/004195 filed Jul. 9, 2010, whichclaims the benefit of European Patent Application No. 09009032.5, filedJul. 10, 2009, all of which are incorporated herein by reference intheir entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to a knee prosthesis and to a method ofmanufacturing components for such a prosthesis. In particular, thepresent invention provides a knee joint prosthesis, as well asindividual components of the prosthesis. In addition, the inventionprovides a method of finishing articulation surfaces of the componentsof the knee joint prosthesis, as well as an apparatus for performingthat method.

BACKGROUND OF THE INVENTION

Knee joint prostheses are typically employed in patients sufferingserious afflictions of the knee joint, e.g. caused by diseases such asosteoarthritis. Such prostheses typically involve a partial or fullreplacement of the knee joint, and the components of the prosthesis arefully implanted within the body of the patient. In other words, theseknee joint prostheses are generally endoprostheses.

Each year over 100,000 knees are endoprosthetically treated in Germanyalone, and of the knee joint prostheses implanted, complicationsassociated with the prostheses arise in a significant proportion ofcases and necessitate re-treatment of the patients. The causes of thecomplications are numerous and include infection, wear and even failureof the prosthesis components. One of the main causes is loosening of theimplant, which occurs in about 26% of cases, and studies have shown thisoften to be directly associated with wear of the implant components.

Conventional knee joint prostheses are typically designed to provide acombination of both hard and soft components. For example, thefunctional surfaces of the prostheses are typically provided bycomponents respectively fabricated from cobalt-chromium alloys (CoCr) onthe one hand, and polyethylene (PE) on the other, thus providing asliding interface between components of these two materials (CoCr-PE).With numerous patients susceptible to allergic reactions from nickel-and cobalt-chromium based alloys, however, titanium (Ti) is oftenemployed as an alternative, such that the prostheses then havetitanium-polyethylene (Ti-PE) material interfaces. A primary reason forthe loss of stability in the anchorage of the implant components andtheir consequent loosening has been indentified to be associated withthe accumulation of polyethylene wear particles in the joint. As aresult of immunologically induced, inflammation-like reactions in thepatient's body, the fragments or particles of polyethylene can activateosteoclasts, which act to remove or degrade bone tissue in the joint.Thus, the wear of the softer polyethylene components has been found toresult in a reduced service life of the endoprosthesis, which is rarelymore than about ten years.

In view of their excellent wear-resistance properties, ceramic materialswould appear to be highly advantageous for use in such endoprostheses.Ceramic materials have the disadvantage, however, that they arerelatively brittle (i.e. they have low fracture toughness), making themhighly susceptible to fracture when stressed. As a result, ceramicmaterials are particularly sensitive to the formation of stressconcentrations.

The geometry of conventional knee joint prostheses is, not surprisingly,based upon the physiological geometry of the human knee joint, whichcomprises femoral and tibial condyles. In this connection, the femoralcondyles may be generally described in the sagittal plane (i.e. anantero-posterior direction) as having two distinct radii. Thus, theradius of that part of the condyles which is in contact in the kneejoint when the leg is in extension is distinctly different to the radiusof that part of the condyles which is in contact in the knee joint whenthe leg is in flexion. The tibial condyles are lightly curved andcombine with the meniscus to form a plateau having a somewhat flattercharacter for receiving the end of the femur. The geometry of thefemoral condyles and the associated tibial plateau result in a specificmotion of the joint, in which the femoral condyles partly slide upon thetibial plateau, but also roll in a rearward or posterior direction athigher angles of flexion. In order to reproduce the kinematics of thehuman knee joint in an endoprosthesis, corresponding radii combinationsare typically incorporated into the prosthesis designs so that acombined sliding and rolling movement is achieved.

One of the reasons why the use of ceramic materials has not been broadlyand successfully implemented in knee joint endoprostheses to date isrelated to the complex geometry and the rolling contact between theprosthesis components. Such rolling contact creates localized loading ofthe contacting surfaces of the prosthesis, which in turn creates stressconcentrations and can lead to component failure in ceramic materials.In addition, the required level of geometric precision and surfacesmoothness is often not achieved in the finishing processes for ceramiccomponents, and unevenness in the ceramic contact surfaces can similarlylead to high stress concentrations under load.

Ceramic components are shaped in a “green” format and are subsequentlysintered. Due to the loss or reduction in the volume of the ceramicmaterial which inevitably occurs during the sintering process, thedimensional variations or tolerances of the component are relativelyhigh, i.e. between about 2% and 5% under DIN 40680 (where DIN isDeutsche Industrienorm). Furthermore, after sintering, the ceramic ishard and can only be processed with specialised tools, e.g.incorporating diamond abrasives. In this regard, also, it will be notedthat highly polished sliding surfaces are required in a knee jointendoprosthesis to ensure that the degree of wear is as small aspossible. Consequently, the requisite degree of precision in thedimensioning and finishing of the ceramic components is simply notpossible without the ceramic material undergoing a machine finishingprocedure.

The kinematics and the relative complex geometries of the human kneejoint, in combination with the requirements of implant quality, haveimpeded the application of ceramic materials in knee jointendoprostheses to date. That is, suitable processing technologies forgenerating and finishing the complex geometries required for apractically viable knee joint prosthesis formed of ceramic material havenot been available. In this regard, basic mechanisms in grinding andpolishing of ceramic free-form surfaces are currently the subject ofon-going research.

Nevertheless, some attempts to develop knee joint prostheses whichemploy ceramic materials have been made. For example, in theInternational Patent Application published as WO-01/30277 A1, a kneejoint endoprosthesis having a ball-shaped femoral component and acorresponding spherical socket in a tibial component is described. Sucha joint, however, provides the articulation of a fixed hinge, which isbiomechanically quite unsuitable for a human knee joint. TheInternational Patent Application published as WO-2006/130350 A2, on theother hand, describes a knee joint endoprosthesis formed from aparticular ceramic material having an especially high fracturetoughness. This prosthesis, however, still suffers from the problem thatthe femoral component is designed to provide a rolling motion relativeto the tibial component, and this leads to stress concentrations betweenthe engaging surfaces.

The present invention is directed to the development of a new andimproved knee joint prosthesis, and in particular to a partial or totalknee joint endoprosthesis, which aims to provide increased service lifeand reduced incidences of component loosening through superior wearproperties. At the same time, the invention aims to provide a knee jointprosthesis which is able to meet increasing patient expectations of goodjoint mobility. The present invention is also directed to a new methodof producing components of a knee joint prosthesis which have desiredsurface characteristics, and to an apparatus for carrying out such amethod.

BRIEF SUMMARY OF THE INVENTION

According to one aspect, the present invention provides a knee jointprosthesis comprising a tibial component and a femoral component. Thetibial component has a fixation portion adapted to be fixed to an upperend of a prepared tibia in a patient, and a bearing portion presentingat least one articulation surface formed from a ceramic material. Thefemoral component is adapted to be fixed to a lower end of a preparedfemur in a patient and has a body portion presenting at least onearticulation surface formed from a ceramic material for engagement withthe respective articulation surface of the tibial component. Thearticulation surfaces of the tibial component and the femoral componentare configured for essentially congruent or conforming engagement withone another over a full range of movement of the prosthesis. Further,the bearing portion of the tibial component is adapted for movementrelative to the fixation portion.

Because the articulation surfaces of the components remain essentiallycongruent or in conformity in their engagement with one another over thefull range of prosthesis movement, the forces between the components aredistributed over the entire area of the surface contact and stressconcentrations at localised areas are avoided. Thus, the area ofengagement or contact of the articulation surfaces remains substantiallyuniform. In this regard, the reference to “congruent” engagement may begenerally understood as a reference to the fact that, over the extent ofits area, the at least one articulation surface of the bearing portionof the tibial component essentially coincides with and maintains uniformcontact with the at least one articulation surface of the body portionof the femoral component. It will be appreciated, however, that the term“congruent” in this sense does not mean that the respective articulationsurfaces have the same extent. Furthermore, by designing the bearingportion of the tibial component for movement relative to the fixationportion, the desired joint kinematics are able to be obtained. In thisregard, the bearing portion is desirably configured for translationalmovement relative to the fixation portion, e.g. in an anterior and/orposterior direction. The bearing portion may also be configured forrotational movement relative to the fixation portion.

Desirably, therefore, the tibial component and the femoral component ofthe knee joint prosthesis permit only relative sliding contact betweenthe articulation surfaces over the full range of movement of theprosthesis, e.g. from a fully extended position to a fully flexedposition of the knee joint prosthesis. The contacting or matingarticulation surfaces thus maintain their congruency or conformity insliding contact over a full range of prosthesis articulation. In thisway, the prosthesis is designed to avoid or eliminate the conventionalrolling contact between the articulation surfaces which, in turn, avoidsor eliminates a significant source of stress concentrations at localisedregions of the articulation surfaces.

In a preferred form of the invention, the articulation surfaces areconfigured to follow a curve in a sagittal plane defined by a constantradius over a full extent of the engagement of the articulationsurfaces. In this regard, the articulation surfaces of the tibial andfemoral components are preferably formed as partially cylindricalsurfaces, or as partially spherical surfaces, or as partially toricsurfaces. The at least one articulation surface of the tibial componentwill typically have a concave profile, while the at least onearticulation surface of the femoral component will typically be formedwith a corresponding convex profile. Accordingly, in the sagittal plane,the articulation surfaces of the tibial and femoral components havecomplementary circular profiles (i.e. partially circular) forcontinuous, sliding engagement with one another over the range ofprosthesis movement.

In one preferred form of the invention, the knee joint prosthesis isuni-compartmental for a partial knee joint arthroplasty, i.e. at justone of the condyles. As such, in this embodiment the bearing portion ofthe tibial component has only one articulation surface, and the bodyportion of the femoral component presents a complementary articulationsurface for engagement therewith. The partial knee joint prosthesis ofthis embodiment may therefore be adapted to the geometry of either themedial or the lateral condyle of the knee.

In an alternative preferred form of the invention, however, the kneejoint prosthesis is designed for a total knee joint arthroplasty, i.e.it is designed to create a bicondylar joint. Thus, in this embodimentthe bearing portion of the tibial component presents two laterallyspaced apart articulation surfaces, and the body portion of the femoralcomponent presents two correspondingly laterally spaced articulationsurfaces for engagement with the respective articulation surfaces of thetibial component. Preferably, the two articulation surfaces of each ofthe tibial and femoral components have separate centres of curvature forthe curve in the sagittal plane, and the respective centres of curvatureare spaced from one another laterally, i.e. in the medio-lateraldirection.

In the knee joint prosthesis of the present invention, both the bearingportion of the tibial component and the body portion of the femoralcomponent are formed of ceramic material. In this connection, theparticular ceramic selected will typically be hard (i.e. low wear) andhave high fracture toughness, and the ceramic will naturally also needto be bio-compatible (i.e. bio-ceramics). Examples of ceramic materialswhich may be used in the prosthesis of the invention include: aluminiumoxide ceramics, such as Al 999 (>99.8% Al₂O₃) according to DIN EN 60672,type C799; zirconium oxide ceramics according to DIN EN 60672, such asZrO₂-TZP, ZrO₂-TZP-A, and ZrO₂-ATZ; as well as mixed ceramics, such aszirconium-reinforced aluminium oxide having e.g. 81% Al₂O₃ and 17% ZrO₂.Such ceramic materials are commercially available under trade names suchas Biolox® forte or Biolox® delta from CeramTec or Bio HIP® fromMetoxit.

According to another aspect, the present invention provides a knee jointprosthesis comprising a tibial component to be attached to an upper endof a prepared tibia in a patient. The tibial component comprises abearing portion formed from a ceramic material and presents at least onearticulation surface which follows a curve in a sagittal plane definedby a constant radius over a substantially full extent of thearticulation surface. The bearing portion is adapted for translationalmovement relative to the upper end of the tibia, preferably in anantero-posterior direction. In this way, the constant radial curvatureof the at least one articulation surface in the sagittal plane isadapted for continuous distributed engagement with a correspondingarticulation surface of a femoral component through an angle of rotationor pivot in that plane, whereby the translational movement of thebearing portion relative to an upper end of the tibia ensures thedesired joint kinematics are achieved. Preferably, the at least onearticulation surface of the tibial component is partially cylindrical,or partially spherical, or partially toric.

In a preferred form of the invention, the knee joint prosthesis furthercomprises a femoral component adapted to be fixed to a lower end of aprepared femur in a patient. The femoral component has a body portionformed from a ceramic material and presents at least one articulationsurface for engagement with the respective articulation surface of thetibial component. In this regard, the articulation surfaces of thetibial and femoral components are desirably configured for essentiallycongruent engagement over a full range of movement of the prosthesis.

As already noted above, in one preferred form of the invention, thebearing portion of the tibial component presents two articulationsurfaces laterally spaced apart from one another. Thus, the body portionof the femoral component preferably presents two corresponding laterallyspaced articulation surfaces for engagement with the respectivearticulation surfaces of the tibial component. The two articulationsurfaces of each of the tibial and femoral components preferably haveseparate centres of curvature, and the respective centres of curvatureare spaced from one another laterally, i.e. in the medio-lateraldirection.

According to a further aspect, the present invention provides a tibialcomponent of a knee joint prosthesis, which tibial component is to beattached to an upper end of a prepared tibia in a patient. The tibialcomponent comprises a bearing portion formed from a ceramic material andpresenting two articulation surfaces spaced from one another in amedio-lateral direction. Each of the articulation surfaces follows acurve in the sagittal plane having a substantially constant radius, andeach of the articulation surfaces has a separate centre of curvature.The centres of curvature are thus spaced from one another in themedio-lateral direction. Preferably, the articulation surfaces of thetibial component are concave surfaces.

In a preferred form of the invention, each of the articulation surfacesis substantially entirely partially cylindrical, whereby the centre ofcurvature in each case comprises an axis. In an alternative preferredform of the invention, each of the articulation surfaces issubstantially entirely partially spherical or partially toric. Whenformed partially spherical or partially toric, the articulation surfaceprovides lateral stability to the knee joint prosthesis, as thisgeometry inhibits relative movement (e.g. sliding movement) between thetibial and femoral components in a lateral (i.e, medio-lateral)direction.

In a preferred form of the invention, the tibial component furthercomprises a fixation portion adapted to be fixed to the upper end of theprepared tibia in the patient. The bearing portion is adapted to besupported on the fixation portion and is adapted for movement relativeto the fixation portion. For example, the bearing portion may be adaptedfor translational movement relative to the fixation portion, e.g. in ananterior and/or posterior direction. Alternatively, or in addition, thebearing portion may be adapted for pivotal or rotational movementrelative to the fixation portion, preferably about a substantiallyvertical axis.

In a preferred form of the invention, the fixation portion of the tibialcomponent comprises a seating member upon which the bearing portion issupported. The bearing portion is preferably adapted for slidingmovement on the seating member. The seating member may, for example, beconfigured as a plate-like member, and an upper surface thereof may forma seating surface upon which the bearing portion is supported and movesrelative to the fixation portion. The seating surface is thereforepreferably substantially flat and smooth. The seating member may also beadapted to seat against or to abut an upper end of the prepared tibia.

The fixation portion of the tibial component typically includes ananchoring member configured to be inserted into a recess formed in anupper end of the prepared tibia. The anchoring member is adapted toextend into the tibia and is usually securely fixed in the recess using,for example, a biocompatible cement. Desirably, also, the anchoringmember is configured to inhibit rotation of the fixation portionrelative to the tibia. In this regard, the anchoring member may comprisea plurality of laterally arranged or laterally extending elements, suchas wings or fins, adapted to interact with sides of the recess formed inthe bone to resist rotation of the fixation portion relative to thetibia. These elements also provide contact surfaces for ensuring goodadhesion to the bone.

In a preferred form of the invention, the knee joint prosthesis furthercomprises joint stabilising means adapted to limit the degree or extentof relative movement between the tibial component and the femoralcomponent. Thus, the stabilising means may form a stop member fordefining a limit to the movement of the prosthesis between a fullyextended position and a fully flexed position. Furthermore, thestabilising means may be adapted to retain the articulation surfaces ofthe tibial and femoral components in registration with one anotherand/or to prevent the components from inadvertently slipping, e.g. in amedial or lateral direction, out of proper engagement or alignment.

In a healthy human knee, medial-lateral and anterior-posterior stabilityis provided to the joint by the collateral and cruciate ligaments incombination with the meniscus. When a patient receives a knee jointprosthesis, however, it is usually necessary to at least partially, ifnot fully, remove the meniscus and at least some of these ligaments.Thus, in the absence of some or all of these structures, the knee jointprosthesis of the invention may be configured with stabilising means toprovide the necessary medial-lateral and/or anterior-posteriorstability. In this connection, the stabilising means may include aprojecting member which, in the case of a full knee joint prosthesis,may be located between the two laterally spaced articulation surfaces.The projecting member is desirably upstanding from the tibial componentand is adapted to be received within an opening or aperture formedbetween articulation surfaces of the associated femoral component of theprosthesis. In this manner, the projecting member of the tibialcomponent cooperates with the opening or aperture in the femoralcomponent to form means for registering the position of the femoralcomponent relative to the tibial component and may also laterallystabilise the position of femoral component relative to the tibialcomponent. Although the projecting member in this embodiment extendsfrom and/or is part of the tibial component, it will be noted thatalternative embodiments in which the projecting member extends from thefemoral component for receipt in an opening or aperture of theassociated tibial component may also be envisaged.

In a preferred form of the invention, the opening or aperture formedbetween the articulation surfaces for receipt of the projecting memberis formed as an elongate channel or slot for receiving or accommodatingthe projecting member throughout the full range of movement of theprosthesis—i.e., during movement of the femoral component relative tothe tibial component between a fully extended position and a fullyflexed position of the joint. Furthermore, the elongate channel or slotmay include or define a limit to the relative movement of the femoralcomponent relative to the tibial component. In this regard, part of theelongate channel or slot (e.g. an end region thereof) may be adapted tocontact or abut the projecting member to stop or prevent furtherrelative movement between the components. Preferably, the engagementbetween the projecting member and the limiting part or end region of thechannel or slot is via surface contact (i.e. as opposed to point or linecontact), again being designed to avoid or to minimise the generation ofstress concentrations.

In a preferred form of the invention, an upper surface of the projectingmember is curved and configured for substantial conformity with an outersurface of the femoral component. An outer surface of the femoralcomponent may therefore be configured to move (e.g. to slide) over theupper surface of the projecting member, at least at an end region of themovement of the joint towards the full flexed position. As noted above,the projecting member may be adapted to contact or abut an end region ofa channel or slot formed in the femoral component to stop or limitrelative movement between the tibial and femoral components at one endregion or extent of movement of the joint. At the other end region orextent of movement, however, the curvature of the upper surface of theprojecting member may allow the femoral component to slide over theprojecting member.

In a preferred form of the invention, the projecting member forms apivot for pivotal or rotational movement of the bearing portion relativeto the fixation portion of the tibial component, e.g. about asubstantially vertical axis. In this regard, the projecting member may,for example, be configured as a stud-like or peg-like member, andpreferably has a curved (e.g. generally cylindrical) outer surface. Itwill be appreciated, however, that other geometries are also possible.

In a preferred form of the invention, the bearing portion includes anaperture or slot for receiving and/or accommodating the projectingmember. The aperture or slot in the bearing portion is preferablyadapted to interact with the projecting member to define an extent orlimit of movement (e.g. translational and/or rotational movement) of thebearing portion relative to the fixation portion of the tibialcomponent. That is, the aperture or slot in the bearing portion may beadapted to contact or abut the projecting member to limit or stop suchmovement relative to the fixation portion. In this connection, forexample, the projecting member may have surface geometries which arecomplementary to surface geometries of the aperture or slot in thebearing portion in order to obtain contact or abutment over a largersurface area and thereby avoid the generation of stress concentrations.

In an alternative preferred form of the invention, the projecting memberis provided on the bearing portion of the tibial component and projectsdownwardly from the bearing portion for interaction with the fixationportion of the tibial component. For example, the fixation portion maycomprise a recess or cavity, such as a channel or slot, for receivingthe projecting member and may define limits or boundaries to movement ofthe bearing portion relative to the fixation portion. In this regard,the sides or ends of the recess or cavity formed in the fixation portionmay be configured to interact with the projecting member to limittranslational or rotational movement of the bearing portion relative tothe fixation portion.

In a preferred embodiment of the invention, the bearing portion includesa base having a lower bearing surface for contact or engagement with thefixation portion. The base of the bearing portion may, for example, haveor support one or more prominences on an upper side thereof. Thearticulation surface(s) may be formed or provided at an upper side ofthe one or more prominences. In one embodiment the base is substantiallyplate-like and flat. In an alternative embodiment, however, the base isoriented (e.g. pitched or set) at an angle in a rearward or posteriordirection such that the articulation surfaces are also pitched or angledin the rearward or posterior direction. The angle at which the base isoriented is preferably in the range of 1° to 30°, and more preferably inthe range of 3° to 15°.

In a preferred embodiment of the invention, the lower surface of thebase of the bearing portion is substantially planar or flat. However,the base may alternatively have a curved lower bearing surface adaptedto essentially congruently engage a curved seating surface on the upperside of the fixation portion, where the lower bearing surface and theseating surface have a common radius in the sagittal plane. In such anembodiment, the base of the bearing portion may optionally have a twopart structure.

According to yet another aspect, the invention provides a method ofproducing ceramic components of a knee joint prosthesis to be implantedin a patient, and more particularly, a method of finishing articulatingsurfaces of ceramic components of a knee joint prosthesis prior toimplantation, the method comprising the steps of:

providing a ceramic bearing portion for a tibial component of a kneejoint prosthesis, the bearing portion presenting at least onearticulation surface; and

providing a ceramic body portion for a femoral component of a knee jointprosthesis, the body portion presenting at least one articulationsurface for engagement with the respective articulation surface(s) ofthe bearing portion of the tibial component;

wherein the articulation surfaces of the bearing portion and the bodyportion of the tibial and femoral components are provided withsubstantially complementary or conforming surface profiles;

combining the bearing portion and the body portion such that therespective articulation surfaces of the components are brought intoengagement with a working substance provided between them; and

lapping the engaging articulation surfaces or imparting relativemovement to the bearing portion and the body portion—i.e. moving thebearing portion and the body portion such that the essentially congruentprofiles of the articulation surfaces repeatedly slide over oneanother—whereby forces generated between the articulation surfaces andthe working substance act to finish the articulation surfacessimultaneously.

In a preferred form of the invention, the method includes forming and/ormachining the bearing portion and the body portion to provide therespective articulation surfaces thereof with substantially congruent orconforming surface profiles. This is naturally performed as apreliminary procedure before the lapping operation.

In a preferred form of the invention, the step of imparting movementinvolves imparting reciprocating relative movement between the bearingportion and the body portion, e.g. over a substantially full range ofarticulation or movement of the tibial and femoral components. In thisregard, the substantially complementary surface profiles are curvedsurface profiles, and the curved profiles of the articulation surfaceshave a common radius in the sagittal plane. Thus, the method involvesthe step of imparting reciprocating rotational movement in the sagittalplane about the centres of curvature of the curved surface profilesbetween the bearing portion and the body portion over a substantiallyfull range of articulation or movement of the tibial and femoralcomponents. Where the articulation surfaces are e.g. cylindrical,however, the reciprocating relative movement between the bearing portionand the body portion may alternatively, or in addition, be provided in alateral (medio-lateral) direction—that, is in a direction along the axisof the cylinder. By combining both rotational and lateral relativemovements, a more uniform surface finish may be obtained. Thereciprocating movement with the inventive method is cyclical andpreferably has a frequency in the range of about 0.5 to about 2 Hz.

In a preferred form of the invention, the working substance is appliedor distributed essentially uniformly between the articulating surfacesof the bearing portion and the body portion. Preferably, the step ofapplying or distributing the working substance comprises: providing areceptacle containing the working substance, and combining the bearingportion and the body portion such that the articulation surfaces of thecomponents are brought into engagement within the receptacle; that is,preferably substantially immersed in the working substance.

In a preferred form of the invention, the working substance has abrasiveproperties and is adapted to grind, to polish, or to buff thearticulating surfaces of the ceramic components. The working substancepreferably comprises a liquid or paste incorporating abrasive particles,such as diamond particles. The method of the invention may include thestep of changing the working substance to achieve stepwise changes inthe degree or level of finish in the articulation surfaces. For example,a working substance containing particles of one basic size or grade(e.g. relatively coarse) may be initially employed to obtain a first orinitial surface finish. The working substance may then be replaced withanother having particles of a second (e.g. finer) grade to achieve asmoother finish. The step of altering the working substance to achieve afiner surface finish may also be carried out more than once. In thisconnection, the particle grades (i.e. notional particle sizes) for usein diamond pastes for finishing such ceramic surfaces include thefollowing basic ranges: 20 μm to 40 μm (i.e. relatively coarse), 1 μm to10 μm (i.e. medium) and 0.5 μm to <0.25 μm (i.e. fine).

In a preferred form of the invention, the method further comprises thestep of applying a predetermined or regulated contact pressure betweenthe bearing portion and the body portion during the step of lapping orimparting relative movement to enhance conformity in the form or profileof the engaging articulation surfaces of the respective components. Inthis connection, the contact pressure may be varied depending on theparticular stage or type of surface finishing being executed. Forexample, during treatment of the articulation surfaces with an abrasiveworking fluid having a relatively coarse particle size, with whichrelatively high material removal is desired, a higher contact pressure(e.g. in the range of 30 N/cm² to 40 N/cm²) may be employed. In suchcases, a material (i.e. thickness) removal at a rate of about 100 μm/minis typical. On the other hand, during treatment of the articulationsurfaces with a working fluid having a relatively fine particle size,with which a higher surface quality (i.e. smoothness) is desired, alower contact pressure (e.g. in the range of 3 N/cm² to 16 N/cm²) isused and a lower rate of material removal of about 1 μm/min to 2 μm/minis achieved.

In a preferred form of the invention, the method includes the step ofusing a working substance (typically a fluid) having no abrasiveparticles. Such a working substance is typically used in a final phaseof surface finishing, after a high level of surface smoothness hasalready been attained. Because no abrasive particles are contained inthe working fluid, higher contact pressures may be employed in thisfinal phase (e.g. contact pressures in the range of about 50 N/cm² to150 N/cm²). Performing the lapping procedure of the invention with sucha working fluid effectively performs a final “run-in” of the ceramicparts of the prosthesis before they are implanted. It also enables theceramic parts to be tested under loads typically experienced in use.

Thus, the method of the invention may include the step of applying apredetermined or regulated force to the bearing portion and the bodyportion (e.g. in a direction substantially perpendicular to thearticulation surfaces) during the step of lapping or imparting relativemovement to generate the desired contact pressure between thearticulation surfaces. The predetermined or regulated force may beconstant or may vary throughout each cycle or reciprocation according toa desired force profile. In this regard, it will be noted that theforces or pressures exerted upon the knee joint in vivo are not usuallyuniform or constant through the range of joint movement. The amount offorce applied in the finishing procedure while using a working substancecontaining abrasive particles is desirably within the range of about 250N to about 1.0 kN. When “running-in” the components of the prosthesiswith a working fluid that does not contain abrasive particles, theamount of force applied is typically within the realms of forcemagnitudes likely to be experienced by the components in use; e.g.within the range of about 1.0 kN to about 3.0 kN. The contact pressuresthat arise between the articulation surfaces under such loads willnaturally depend upon the contact area of the articulation surfaces inthe actual prosthesis concerned, but for a contact area of about 20 cm²then contact pressures in the range of about 50 N/cm² to 150 N/cm² maybe expected.

As a result of the complementary or conforming geometry of thearticulation surfaces in the knee joint prosthesis of the invention, andthe fact that these surfaces are formed in ceramic components, themethod of the invention makes it possible to finish the articulationsurfaces to such a high degree of conformity and congruence that a“run-in” period for the prosthesis no longer required once implanted inthe patient. Indeed, the method of the invention has been able toproduce a degree of congruence and a low-wear surface finish or surfacequality that other surface finishing techniques to date have not beenable to achieve with the geometries of a knee prosthesis. This surfacefinish exhibits, for example, a micro-geometric surface topography thatis almost isotropic and has demonstrated extremely good tribology andwear characteristics.

Thus, with the above method, the bearing portion of the tibial componentand the body portion of the femoral component are fully adapted to oneanother prior to implantation and have mating surfaces in a degree ofconformity with one another not previously realisable. In this regard,the quality of the surface finish may provide the following: a formprecision of 1 μm/m, a middle roughness value Ra<0.3 μm, the avoidanceof surface waviness (i.e. <4 μm), and roundness of to 0.2 μm. And whenconsidering the geometry of the articulation surfaces formed in theceramic bearing portion and the ceramic body portion as a whole, theinventive method is able to provide diameter tolerance in the range of 0to 50 μm (typically 10 μm), spherical deviation ≦5 μm, accuracy of fitof the components ≦1 μm, and roughness value 0.01<Ra<0.3 μm.

According to still a further aspect of the present invention, anapparatus for carrying out or performing the above-described method offinishing the articulating surfaces of ceramic components of a kneejoint prosthesis is also provided. In particular, the invention providesan apparatus comprising: a frame; a first holding device, such as aclamp or chuck, for mounting and securely holding a ceramic bearinginlay of the knee joint prosthesis, the bearing inlay having at leastone articulation surface; and a second holding device, such as a clampor chuck, for mounting and securely holding a ceramic body of thefemoral component of the knee joint prosthesis, the ceramic body havingat least one articulation surface. At least one of the first and secondholding devices is provided on the frame so as to be adjustablypositionable relative to the other holding device to bring therespective articulation surfaces of the ceramic bearing inlay and theceramic body into engagement with one another when the bearing inlay andthe body are held by the respective holding devices. Furthermore, theapparatus include means for moving at least one of the first and secondholding devices such that the ceramic bearing inlay and/or the ceramicbody move relative to one another when the articulation surfaces are inengagement, such that the articulation surfaces repeatedly slide overone another. The moving means is preferably adapted to rotate the firstand/or second holding device about a centre of curvature of the at leastone articulation surface of the ceramic bearing inlay and/or the ceramicbody held therein when the respective articulation surfaces are inengagement.

In a preferred form of the invention, the moving means is adapted torotate the second holding device and the ceramic body held therein aboutthe centre of curvature of the ceramic body's at least one articulationsurface relative to the ceramic bearing inlay held in the first holdingdevice. It will be appreciated however that, alternatively or inaddition, the moving means may also be adapted to rotate the firstholding device and the ceramic bearing inlay held therein about thecentre of curvature of the inlay's at least one articulation surfacerelative to the ceramic body held in the second holding device. Therelative rotation which thereby results between the ceramic componentsis a reciprocal rotation over the full range of movement of the kneeprosthesis.

In a preferred form of the invention, the apparatus includes areceptacle or container for holding a volume of a working substance,such as a polishing fluid. In this way, the receptacle or container maythereby form a tank or bath for the working fluid. The first and/orsecond holding device for holding the ceramic bearing inlay and/orceramic body is arranged in the receptacle or container. In this way,the ceramic bearing inlay and/or ceramic body may be immersed in theworking fluid while being held by the respective holding device.

In a preferred form of the invention, the second holding device isadapted for adjustable one-, two-, or three-dimensional positioningrelative to the first holding device. For example, the second holdingdevice may be adjustably moveable or positionable on the frame via oneor more displacement mechanism, such as one or more screw mechanism(e.g. hand- or machine-driven screw thread) or hydraulic ram. In thisway, the articulation surfaces of the ceramic bearing inlay and theceramic body held in the holding devices may be aligned with each otherand/or moved towards and away from each other and brought into and outof engagement with one another with a high degree of precision andadjustment.

In a preferred form of the invention, the apparatus is adapted ordesigned to apply a predetermined or regulated force to generate aspecific contact pressure between the engaging articulation surfaces ofthe ceramic inlay and the ceramic body to enhance the finishing thereof.The contact pressure is desirably applied substantially uniformly overthe articulation surfaces.

In contrast to revision knee prostheses, which are typically onlyemployed after an initial knee prosthesis has failed and/or when all thenatural structures and ligaments of the knee no longer function, theprosthesis of the present invention is generally configured to beimplanted with relatively little preparation of the patient's femur andtibia. Thus, whereas the femoral component of a revision knee prosthesiswill typically have long anchors and/or a coupling arrangement forcoupling to the tibial component that requires extensive excavation andpreparation of the patient's femur, the femoral component in the jointprosthesis of the present invention is desirably configured for minimalremoval of bone tissue—preferably replacing just the femoral cartilageand a small amount of bone material required for providing (e.g.flattened) seating surfaces and/or for receiving a short positioningstud. In this way, the femur component in the knee prosthesis of theinvention consists of an external cover component (i.e. presenting thearticulation surface(s)) for the end of the femur and requires little orno excavation the femur bone tissue. Thus, implantation of a knee jointprosthesis according to the present invention involves removal ofminimal bone tissue, which not only simplifies the surgical procedurebut also reduces the trauma for the patient and improves the patientrecovery time.

For assistance in understanding the present invention, the terms“upper”, “lower”, “above”, “below”, “side”, “lateral”, “laterally”,“medial” “front”, “rear”, “anterior”, “posterior”, “medio-lateral”,“antero-posterior”, “vertical”, and other similar terms used herein inrespect of various parts of the knee joint prosthesis of the inventionare intended to be given their ordinary meaning in view of the normal orin-use orientation of the knee joint prosthesis described herein. Itwill be appreciated, however, that other interpretations of these termsmay be appropriate depending on the particular orientation of theprosthesis and/or its respective parts at the time.

In addition, it will be understood that the use of the term “sagittalplane” herein is not a reference to the mid-sagittal plane which dividesthe human body centrally into two halves, but rather to a para-sagittalplane which passes through the middle of the knee and extends parallelto the mid-sagittal plane.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The above and further features and advantages of the invention willbecome more readily apparent from the following detailed description ofpreferred embodiments of the invention with reference to theaccompanying drawings, in which like reference characters identify likefeatures, and in which:

FIG. 1 is an exploded side perspective view of a knee joint prosthesisaccording to an embodiment of the invention;

FIG. 2 is an exploded rear perspective view of the knee joint prosthesisof FIG. 1;

FIG. 3 is a schematic front perspective view of the knee jointprosthesis of FIG. 1 in an assembled and implanted state;

FIG. 4 is a schematic, partially sectional, front-side perspective viewof the knee joint prosthesis of FIG. 1 in an assembled and implantedstate;

FIG. 5 is a schematic, partially sectional, rear-side perspective viewof the knee joint prosthesis of FIG. 1 in an assembled and implantedstate;

FIG. 6 is a schematic side (sagittal plane) view of the knee jointprosthesis of FIG. 1 in an assembled and implanted state and shown in anextended position;

FIG. 7 is a schematic side (sagittal plane) view of the knee jointprosthesis of FIG. 1 in an assembled and implanted state and shown in apartially flexed position;

FIG. 8 is a side perspective view of the knee joint prosthesis of FIG. 1in an assembled state (but not implanted) and shown in a fully flexedposition;

FIG. 9 is a front perspective view of the fixation portion of the tibialcomponent of the knee joint prosthesis of FIG. 1;

FIG. 10 is a top or plan view of the bearing portion of the tibialcomponent of the knee joint prosthesis of FIG. 1;

FIG. 11 is a side-rear perspective view of the bearing portion of thetibial component of the knee joint prosthesis of FIG. 1;

FIG. 12 is an exploded front perspective view of a tibial component fora knee joint prosthesis (e.g. as shown in FIG. 1) according to anembodiment of the invention;

FIG. 13 is a front perspective view of a tibial component for a kneejoint prosthesis (e.g. as shown in FIG. 1) according to the invention inan assembled state;

FIG. 14 is a top or plan view of the tibial component in FIG. 13;

FIG. 15 is a top or plan view of the tibial component in FIG. 13 showinga posterior or rearward translation of the bearing portion relative tothe fixation portion;

FIG. 16 is a top or plan view of the tibial component in FIG. 13 showinga rotation of the bearing portion relative to the fixation portion;

FIG. 17 is a schematic side (sagittal plane) view of the knee jointprosthesis of FIG. 1 in an assembled and implanted state and shown in afully extended position;

FIG. 18 is a schematic side (sagittal plane) view of the knee jointprosthesis of FIG. 1 in an assembled and implanted state and shown in afully flexed position;

FIGS. 19a to 19c are sectional views of a knee joint prosthesisaccording to different embodiments of the invention taken in amedio-lateral direction;

FIG. 20 is a schematic and partially sectional side view of a knee jointprosthesis according to another embodiment of the invention in anassembled and implanted state and shown in a fully extended position;

FIG. 21 is a schematic and partially sectional side view of the kneejoint prosthesis of FIG. 20 in an assembled and implanted state andshown in a fully flexed position;

FIG. 22 is a partial and sectional perspective side view of a bearingportion of the tibial component of the knee joint prosthesis of FIG. 20;

FIG. 23 is a schematic side view of a knee joint prosthesis according toa further embodiment of the invention in an assembled and implantedstate and shown in a fully flexed position;

FIG. 24 is a schematic and partially sectional side view of a knee jointprosthesis according to the embodiment of FIG. 23 in an assembled andimplanted state and shown in a fully flexed position;

FIG. 25 is a schematic perspective view of a knee joint prosthesis in anassembled state according to another embodiment of the invention;

FIG. 26 is a cross-sectioned perspective view of the knee jointprosthesis in FIG. 25;

FIG. 27 is a side perspective view of the tibial component of theprosthesis of FIG. 25;

FIG. 28a is a sectioned side view (sagittal plane) of the knee jointprosthesis of FIG. 25 shown in a position representing near fullextension of the leg;

FIG. 28b is a side view of the tibial component shown in FIG. 28 a;

FIG. 29a is a sectioned side view (sagittal plane) of the knee jointprosthesis of FIG. 25 shown in a position representing full flexion ofthe leg;

FIG. 29b is a side view of the tibial component shown in FIG. 29 a;

FIG. 30 is a schematic side perspective view of a knee joint prosthesisaccording to yet a further embodiment of the invention in an implantedstate;

FIG. 31 is a schematic side perspective view of the knee jointprosthesis in FIG. 30 in an assembled but non-implanted state;

FIGS. 32a, 32b and 32c are top and cross-sectional perspective views ofa tibial component of a uni-compartmental knee joint prosthesis of theinvention;

FIGS. 33a and 33b show different positions of the inlay on the tibialcomponent;

FIGS. 34a, 34b, 34c and 34d are perspective views of variants in theform of the tibial component of a uni-compartmental knee jointprosthesis of the invention;

FIG. 35 is a schematic front view of an apparatus for finishing aceramic bearing portion and a ceramic body portion with a preferredmethod of the invention;

FIG. 36 is a schematic partial view of the apparatus in FIG. 35 forfinishing a ceramic bearing portion and a ceramic body portion with amethod of the invention; and

FIG. 37 is a schematic side view of the apparatus shown in FIG. 36.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The Referring firstly to FIGS. 1 and 2 of the drawings, the primarycomponents of a knee joint prosthesis 1 according to a preferredembodiment of the invention are shown in exploded perspective views. Asis clearly apparent, the knee joint prosthesis 1 in this embodiment iscomprised of three discrete or separate parts. The lower part and themiddle part in FIGS. 1 and 2 combine to form a tibial component 2 forattachment at an end of a tibia T of a patient, while the upper partforms a femoral component 3 which is designed to be attached at an endof a femur F of the patient.

With reference to FIGS. 3 to 5 of the drawings, the implanted or in-useorientation and interrelationships of the tibial component 2 and thefemoral component 3 of the knee joint prosthesis 1 of the invention areillustrated. Furthermore, in addition to the tibia T and femur F, thepatella P (commonly called the “knee cap”) is illustrated in relation tothe components of the prosthesis.

Considered in more detail, the lower part of the tibial component 2comprises a fixation portion 10 which is adapted to be fixed at theupper end of the prepared tibia T. In this regard, the fixation portion10 comprises an anchoring member 12 which is designed to be insertedinto a corresponding recess formed in the end of the prepared tibia T,and a seating member 14 having a plate-like configuration which isadapted to seat against a resected and flattened end of the preparedtibia T above the elongate anchoring member 12. The upper part of thetibial component 2, on the other hand, comprises a bearing portion 30which is formed of a ceramic material and is designed to be supported onthe fixation portion 10. The bearing portion 30 is desirably formed asan integral or unitary element and presents two upwardly-facing,concavely curved articulation surfaces 32, which are laterally spacedapart from one another. The bearing portion 30 is sometimes known inthis field as an inlay or an insert, so this part of the tibialcomponent 2 will also be referred to herein as the inlay or bearinginlay 30.

With further reference to FIGS. 1 to 5 of the drawings, the femoralcomponent 3 comprises an integral or unitary body portion 50 formed of aceramic material and having a generally cupped configuration defining acavity or hollow 52 for receiving the end of the prepared femur F. Inthis connection, the cavity or hollow 52 of the ceramic body 50 isencompassed by a number of essentially planar faces 54 on an inner sideof the body for contact with correspondingly resected or flattened sidesof the prepared femur F. The outer side of the ceramic body 50 facingthe bearing inlay 30 of the tibial component 2 includes a pair ofconvexly curved articulation surfaces 56 which are laterally spacedapart in the medio-lateral direction for matching or mating engagementwith the articulation surfaces 32 of the bearing inlay 30. An opening oraperture 58 is provided through the ceramic body 50 and between the pairof laterally spaced apart articulation surfaces 56, and this opening oraperture 58 extends in the form of a channel or slot substantiallyparallel to the articulation surfaces 56.

The articulation surfaces 32, 56 of the tibial and femoral components 2,3 of the knee joint prosthesis 1 essentially correspond to and replacethe physiological condyles of a healthy knee joint, at which thearticulating movement of the joint takes place. In this connection, thearticulation surfaces 56 of the ceramic body 50, i.e. of femoralcomponent 3, are configured for essentially conforming or congruentengagement with the articulation surfaces 32 of the tibial component 2over a full range of movement of the prosthesis 1. In other words, theconcave curvature of each of the articulation surfaces 32 in the ceramicbearing inlay 30 of the tibial component 2 corresponds to and is inconformity with the convex curvature of the articulation surfaces 56formed in the ceramic body 50 of the femoral component 3 throughout thejoint movement. In this way, the articulation surfaces 32 of the bearinginlay 30 remain essentially completely and continuously in contact overtheir extent with the articulation surfaces 56 of the body 50 as thefemoral component 3 rotates through a full range of movement of theprosthesis, i.e. between a fully extended position of the patient's legand a fully flexed position of the patient's leg. This aspect of theinvention will become clearer from the following description.

The continuous essentially full surface contact which is maintained overthe extent of the articulation surfaces 32 of the tibial component 2 isachieved in that the respective articulation surfaces 32, 56 of both theceramic inlay 30 and the ceramic body 50 follow a curve in a sagittalplane which is defined by a constant radius over a full extent of theengagement of those articulation surfaces 32, 56. That is, with the kneejoint prosthesis 1 of the invention, as the lower leg pivots or movesbetween the fully extended position and the fully flexed position, thearticulation surfaces of the tibial component 2 and the femoralcomponent 3 move relative to one another by rotating in the sagittalplane around common centres C. As a result, only sliding contact ispermitted between the articulation surfaces 32, 56 over the full rangeof movement of the prosthesis 1. In this way, rolling motion of thefemoral component, which is typical in conventional knee jointprostheses, is excluded from this embodiment of the invention.

Importantly, however, the knee joint prosthesis 1 in this embodiment isnot merely designed to operate as a fixed hinge, as this would not beconsistent with the normal kinematics of a human knee joint.Accordingly, this knee joint prosthesis 1 also provides for movement ofthe bearing inlay 30 relative to the fixation portion 10 of the tibialcomponent 2 during flexion and extension of the patient's leg. In thisway, a more natural movement of the prosthetic joint can be generated.To further explain this aspect, reference is now made to FIGS. 9 to 12of the drawings, which illustrate the particular configuration andarrangement of the parts of the tibial component 2 in this embodiment ofthis invention.

In drawing FIG. 9, the fixation portion 10 of the tibial component 2 isshown in more detail. As can be seen, the fixation portion 10 in thisembodiment is formed as an integral or unitary part and, like the inlay30, may also be formed from a ceramic material. As the skilled personwill appreciate, however, other materials, e.g. metals, such ascobalt-chromium alloys or titanium, or plastics, such as HDPE, could beemployed in all or part of the fixation portion 10. Thus, the fixationportion 10 need not necessarily be formed as an integral or unitarypart. The anchoring member 12 includes a central stem 11 and laterallyextending fins or wing-like elements 13 which together subtend an angleθ of less than 180° there-between. As noted above, when the anchoringmember 12 is inserted into the recess formed in the upper end of theprepared tibia T, it is typically adhesively rigidly affixed to thetibia using a bio-compatible cement. The side surfaces of the fins orwing-like elements 13 of the anchoring member 12 thus provide for goodadhesion to the cement, and also act to inhibit rotation of the fixationportion 10 relative to the tibia. In this connection, the recess formedin the tibia for receiving the anchoring member 12 may be shaped to atleast partially conform with a cross-sectional profile of the anchoringmember.

As can also be seen in FIG. 9 and FIG. 12 of the drawings, the bearinginlay 30 of the tibial component 2 is supported upon the generallyplate-like seating member 14 of the fixation portion 10. In this regard,an upper surface 15 of the seating member 14 forms a seating surfaceupon which the inlay 30 is adapted to move relative to the fixationportion 10 during use. The seating member 14 also seats against theupper end of the tibia bone when the fixation portion is cemented inposition. In addition to forming a recess for insertion of the anchoringmember 12, the surgeon typically resects the upper end of the bone toform at a flattened area against which the plate-like seating member 14comes into contact during the implantation procedure. Thus, theassociated surgical procedures essentially involve “preparation” of thetibia bone for receiving and attaching the tibial component 2.

With further reference to FIGS. 9 to 13 of the drawings, the fixationportion 10 can be clearly seen to include a projecting member 16upstanding approximately centrally of the plate-like seating member 14.The projecting member 16 has a substantially circular-cylindrical shapeand is designed to cooperate with the bearing inlay 30 as described inthe following. The bearing inlay 30 is formed as an integral ceramicelement and, in this embodiment, has a relatively low profile—i.e. arelatively low thickness in the vertical direction. In this regard, theinlay 30 includes a base 34 to which load imparted at the articulationsurfaces 32 is transferred. The base 34 of the inlay 30 in this case hasa substantially plate-like structure and includes a flat and smoothlower surface 35 which is adapted to be slidably supported upon theseating surface 15 at the upper side of the seating member 14.Furthermore, the inlay 30 has two upwardly directed prominences 36 whichextend from the base 34 and present the articulation surfaces 32 atrespective upper sides thereof. The upward prominences 36 from the base34 form load transfer regions for transferring loads imparted to thearticulation surfaces 32 of the inlay 30 to the fixation portion 10 and,in turn, to the tibia T. Between the prominences 36, an aperture 37 isprovided through the base 34 of the bearing inlay. The aperture 37 isgenerally formed as an elongate slot having substantially straight sides38 and rounded end regions 39, and this slot 37 extends in theantero-posterior direction between the articulation surfaces 32 of theinlay. As seen in FIGS. 12 and 13 of the drawings, the aperture 37 inthe bearing inlay 30 is specifically designed to receive the projectingmember 16 of the fixation portion 10, such that the projecting member 16extends upwardly between the articulation surfaces 32.

As shown in FIGS. 13 to 16 of the drawings, the bearing inlay 30 isdesigned for movement relative to the fixation portion 10 of the tibiacomponent 2. In FIGS. 14 and 15, for example, the potential fortranslational movement of the inlay 30 in an anterior or posteriordirection relative to the fixation portion 10 is clearly illustrated. Inthis regard, it will be noted that an edge of the seating member 14 ofthe fixation portion 10 is shown in dashed or broken lines. Thus, theelongate aperture or slot 37 formed centrally in the bearing inlay 30allows forward and rearward translational movement as indicated by thearrow A in FIG. 15. To this end, the seating surface 15 on the upperside of the fixation portion 10 and the lower surface 35 of the ceramicinlay 30 are both finished (i.e. by machining and/or polishing) to ahigh degree of smoothness to ensure low friction and low wearproperties. As these surfaces 15, 35 are both essentially flat orplanar, an extremely high quality surface finish can be achieved withconventional machining and/or polishing and buffing techniques.

When the bearing inlay 30 slides in the forward direction as far aspossible (i.e. as shown in FIG. 14), the curved posterior or rearwardend 39 of the aperture or slot 37 engages with the rearward or posteriorside of the projecting member 16, which in turn limits or stops thefurther displacement of the inlay 30 relative to the fixation portion10. Similarly, when the bearing inlay 30 is shifted in a rearwarddirection (i.e. as show in FIG. 15), the curved anterior or forward end39 of the aperture or slot 37 engages with a forward or anterior side ofthe projecting member 18, which again limits or stops furtherdisplacement. In this connection, it will be noticed from FIGS. 9, 12and 13 that each of the anterior and posterior sides of the projectingmember 16 includes a tapered protrusion 18 for engagement with the inneredge of the aperture or slot 37. The respective forwardly or rearwardlydirected end face 19 of the tapered protrusions 18 is curved and has asubstantially corresponding profile or surface radius as the curved ends39 of the aperture or slot 37. In this way, when the end faces 19 of theprotrusions 18 come into engagement with the respective curved endregions 39 of the slot 37, the forces transmitted between them are welldistributed and stress concentrations are essentially avoided.

Furthermore, it will be noted that the protrusions 18 have tapered orangled flanks or side surfaces 20. The purpose or function of theseangled flanks or side surfaces 20 becomes clear with reference to FIG.16, in which the bearing inlay 30 can be seen to be adapted forrotational movement relative to the fixation portion 10 of the tibialcomponent 2. That is, the inlay 30 is able to pivot or rotate around theprojecting member 16 about a substantially vertical axis. The tapered orangled protrusions 18 at the front and rear sides of the projectingmember 16 again form abutments which are designed to limit the extent ordegree of the rotation of the inlay relative to the fixation portion.That is, the angled flanks or side surfaces 20 of the protrusions 18engage the straight sides 38 of the aperture 37 formed in the bearinginlay 30 to stop further relative rotation. As will be appreciated,these abutments operate not only for rotations of the inlay in ananti-clockwise direction as shown in FIG. 16, but equally also forrelative rotations of the bearing inlay 30 in the clockwise-direction.The abutment surfaces 20 on the flanks of the protrusions 18 aresubstantially straight such that they are again designed for extensivesurface contact with the straight sides 38 of the slot 37, therebyensuring force distribution over the whole surface area and minimizingstress concentrations.

Returning now to FIGS. 6 to 8 of the drawings, the operation andrelative positions of the various parts of the knee joint prosthesis 1are shown at different stages within the range of joint movement from anextended leg position to a fully flexed position. In FIG. 6 and FIG. 7of the drawings, the tibia T, the femur F, and the patella P, as well asthe associated quadriceps tendon Q and patella ligament L of the patientare shown schematically and partially represented. In drawing FIG. 6,the leg of the patient is in a substantially extended position, asindicated by the alignment with the vertical axis V, shown in brokenlines. The articulation surfaces 32 of the bearing inlay 30 are incongruent engagement with the corresponding articulation surfaces 56 ofthe body 50 of the femoral component 3, with each of the matingarticulation surfaces 32, 56 sharing a common radius R in the sagittalplane. This radius R is desirably selected to be within the range ofabout 10 mm to about 60 mm, and more preferably in the range of about 20mm to about 40 mm. Furthermore, it will be noted that each of thearticulation surfaces 32 of the bearing inlay 30 typically has a surfacearea in the range of about 2 cm² to about 20 cm², and more preferably inthe range of about 6 cm² to about 12 cm². The area of each articulationsurface 32 in this example is about 10 cm² and it makes contact witheach articulation surface 56 of the ceramic body 50 over an angularextent in the sagittal plane of about 80°, although this angle may liein the range of about 30° to 120°, preferably 60° to 100°.

As the patient flexes its leg, as represented by the transition todrawing FIG. 7 showing the femur F in a position rotated in theposterior direction relative to the tibia T, the congruently engagingarticulation surfaces 32, 56 slide relative to one another around theirrespective centres of curvature C in the sagittal plane. As evident fromFIG. 7, however, the centres of curvature C of the articulation surfaces32, 56 also experience a displacement in the posterior directionrelative to the vertical axis V, as indicated by the arrow A′. In thisregard, the bearing inlay 30 moves in the posterior direction (i.e. asseen in FIG. 15), thereby enabling the prosthesis 1 to accommodatekinematics analogous to a healthy knee joint. In the fully flexedposition shown in drawing FIG. 8, the end of the range of movement isreached when the femoral component 3 comes into abutment with theprojecting member 16 upstanding between the articulation surfaces 32 ofthe tibial component 2. The associated stabilizing effect of theprojecting member 16 on the operation of the knee joint prosthesis 1will be described in more detail below.

With reference to FIGS. 17 and 18 of the drawings, the means by whichthe knee joint prosthesis 1 of this embodiment is stabilized in theabsence of many of the natural structures (such as meniscus and cruciateligaments) that ordinarily perform that role can be understood. Indrawing FIG. 17, for example, the leg of a patient having the knee jointprosthesis implanted is shown in a fully-extended (i.e. slightlyhyper-extended) position, such that the femur F is rotated to a positiona few degrees anterior or forwardly of the vertical axis V. The centreof curvature C and the radius R of the congruently engaging articulationsurfaces 32, 56 of the prosthesis 1 are illustrated. As notedpreviously, the ceramic body 50 of the femoral component 3 includes anelongate curved channel or slot-like opening 58 formed between thearticulation surfaces 56. This elongate channel 58 in the ceramic body50 is designed to receive an upper end region of the stud- or peg-likeprojecting member 16, as is also apparent from FIG. 8. In this regard,the channel-like opening 58 has a lateral dimension or width foraccommodating the projecting member 16 in an easy fit, but with littleplay. In this manner, the channel-like opening 58, which extends alongthe antero-posterior arc of the ceramic body 50, facilitatesregistration of the femoral component 3 relative to the tibial component2 in a medio-lateral direction, and the end of the stud- or peg-likeprojecting member 16 received or accommodated within the channel 58inhibits displacement of the ceramic body 50 relative to the tibialcomponent 2 in a medio-lateral direction.

Further, the end regions 59 of the channel 58 formed in the ceramic body50 are rounded or curved for substantial conformity with a correspondingside of the projecting member 16 and define end-points or limits to therange of movement of the femoral component 3 relative to the tibialcomponent 2. Thus, in the fully-extended position shown in FIG. 17, afront or anterior side of the stud- or peg-like projecting member 18abuts against a correspondingly curved inner surface 59 at an anterioror forward end of the channel or slot 58 in the ceramic body 50. At thismoment, also, the bearing inlay 30 can be seen to be in a most forwardor anterior position relative to the fixation portion 10. When the femurF rotates relative to the tibia T through an angle of over 90° (e.g. dueto the patient moving from a standing position to a sitting position),the knee joint prosthesis 1 of this embodiment moves through the fullrange of articulation from the fully extended position to the fullyflexed position (i.e. typically an angular range of between about 95°and 110°). At this position, the posterior side of the projecting member16 engages the curved posterior end 59 of the channel or slot 58 formedin the ceramic body 50 of the femoral component 3. As such, theprojecting member 16 again forms a stop which interacts with the end 59of the channel 58 to limit the degree of rotation of the knee jointprosthesis in the sagittal plane, and thus limit the extent of theengagement of the articulation surfaces 32, 56. It will also beobserved, that in the fully flexed position shown in FIG. 18, thebearing inlay 30 is now displaced in the rearward or posterior directionrelative to the fixation portion 10 shown in FIG. 15, such that a centreof curvature C of the articulation surfaces 32, 56 is also displacedrearwardly, as shown by arrow A. It will also be noticed that thefemoral component 3 includes a groove or elongated depression 51 formedin an upper anterior side of the ceramic body 50 leading into the curvedforward or anterior end 59 of the channel or slot-like opening 58. Thegroove 51 is designed to receive and at least partially accommodate thepatella P and the quadriceps tendon Q as the joint prosthesis flexestoward the position in FIG. 18.

FIGS. 19a to 19c of the drawings show three alternative embodiments of aknee joint prosthesis 1 according to the invention. The primarydifference between each of the embodiments in these drawing figuresrelates to the configuration of the respective articulation surfaces ofthe femoral and tibial components 2, 3. In the embodiment of FIG. 19a ,for example, the respective articulation surfaces 32, 56 of thecomponents are formed substantially entirely as partially cylindricalsurfaces, with a central axis X of the partial cylinder illustrated andspaced at the radius R shown. The centre of curvature C for each pair ofthe medial and lateral articulation surfaces 32, 56 is separate anddistinct and lies along part of the axis X. The upstanding projectingmember 16 which fits snugly within the channel or slot 58 between thelaterally spaced pairs of the engaging articulation surfaces 32, 56 thusprovides lateral stability and inhibits the femoral component 3 fromslipping or displacing in a lateral direction relative to the tibialcomponent 2.

Drawing FIG. 19b , on the other hand, shows a configuration in whicheach of the laterally spaced articulation surfaces 32, 56 on either sideof the projecting member 18 is substantially entirely defined by apartially spherical surface having a radius R and a centre of curvatureC. Again, the centre of curvature C is separate and distinct for themedial and lateral articulation surfaces in this embodiment. Further,because the surfaces are spherical (i.e. rather than cylindrical, as inFIG. 19a ), the mating engagement of the articulation surfaces 32, 56itself acts to inhibit lateral or sideways movement of the femoralcomponent 3 relative to the tibial component 2. Nevertheless, in thiscase, the projecting member 16 still provides additional lateralstabilization. In the embodiment of FIG. 19c , the concave recessesformed by the articulation surfaces 32 in the bearing inlay 30 are quitepronounced and receive the correspondingly convex articulation surfaces56 of the ceramic body 50. In this case, the medial and lateralarticulation surfaces have a relatively small radius r in a transverseplane, but still have a common radius R in the sagittal plane, with eachagain having its own centre of curvature C. When the radius R in thesagittal plane is larger than the radius r in the transverse plane, thearticulation surfaces 32, 56 then have an overall toric configuration.

With reference now to FIGS. 20 to 22 of the drawings, a furtherembodiment of the knee joint prosthesis 1 is illustrated. Thisembodiment differs to the embodiments of FIGS. 1 to 19 in that thebearing inlay 30 of the prosthesis is configured such that thearticulation surfaces 32 are pitched or raked at an angle in therearward or posterior direction. In particular, the base 34 of thebearing inlay 30 is pitched or angled in the rearward direction toproduce an enhanced range of movement, and particularly an enhancedflexion, in the knee joint prosthesis 1. The angle a at which the inlaybase 34 is set is preferably within the range of 1° to 30°, and morepreferably in the range of 3° to 15°. In essentially all other respects,this embodiment is consistent with the other embodiments describedpreviously.

FIGS. 23 and 24 of the drawings illustrate a somewhat more complexembodiment of the knee joint prosthesis 1 according to the invention, inwhich the bearing inlay 30 has a two-part construction, and incorporatesa pitched or angled base part 34 as shown in the previous embodiment,combined with a sub-base 40. The sub-base part 40 has a curved lowerbearing surface 41 which essentially congruently engages with a curvedseating surface 15 on the upper side of the seating member 14. Thecurved seating surface 15 and the curved lower bearing surface 41 bothhave a common radius R_(B) in the sagittal plane and are desirablyeither partially spherical or partially cylindrical surfaces. The centreC_(B) of the common radius R_(B) is preferably located on an axis Bwhich is aligned with a front or anterior edge or rim 22 of the tibialcomponent 2; that is, with a front or anterior edge 22 of seating member14.

In essence, the embodiment of FIGS. 23 and 24 has the advantage that,regarding the antero-posterior direction, the bearing inlay 30 no longermoves in a translational displacement relative to the fixation portion10 of the tibial component 2, but rather in rotational displacementabout the axis B. This rotation, combined with the pitched or angledconfiguration of the base 34, further enhances the degree of flexionavailable with the prosthesis. However, because the curvature R_(B) ofthe essentially congruently engaging surfaces 15, 41 prohibits rotationof the sub-base 40 about a substantially vertical axis V (i.e. aroundthe projecting member 16) relative to the fixation portion 10, atwo-part structure for the bearing inlay 30 is proposed. Accordingly,the base 34 with the prominences 36 presenting the articulation surfaces32 is proposed to be formed as one integral part, and the sub-base 40 isformed as a second integral part. The flat lower surface 35 of the base34 is thus able to slide on a correspondingly flat upper surface 42 ofthe sub-base 40. In this way, the upper part of the inlay 30 (i.e.incorporating the base 34) is still able to pivot or rotate around theprojecting member 16 by rotating relative to the sub-base 40.

With reference to FIGS. 25 to 27 of the drawings, a slightly modifiedembodiment of the invention will now be described. This embodiment issimilar to the embodiment described with reference to FIGS. 1 to 7 ofthe drawings, with the primary difference relating to the configurationof the projecting member 16 extending upwardly from the centre of theplate-like seating member 14 in the fixation portion 10 of the tibialcomponent. In particular, it will be noted that an upper surface 23 ofthe projecting member 16 is concavely curved and that the projectingmember 16 is itself not quite as prominent as in the first embodiment.With reference to FIG. 26, it will also be noted that projecting member16 is not formed unitarily with the seating member 14 or the anchormember 12 of the fixation portion 10, but rather is a separate elementwhich is rigidly fixed in a central region at the upper side of theseating member 14. The fixation is via one or more stud-like protrusion24 which may be received in a corresponding recess formed in the seatingmember 14 and/or via adhesive bonding. As was the case in the firstembodiment, the bearing inlay 30 includes an elongate central slot oraperture 37 within which the projecting member 16 is received when theinlay is positioned on the fixation portion 10. This slot or aperture 37in the inlay cooperates with the projecting member 16 in the mannerdescribed with respect to the first embodiment to define the limits oftranslational and rotational displacement of the inlay 30 on thefixation portion 10 during movement of the joint.

In this embodiment, however, the curved upper surface 23 of projectingmember 16 provides for a somewhat modified operation of the prosthesiscompared to the first embodiment, and the nature of this modifiedoperation can be understood with reference to FIGS. 28 and 29 of thedrawings. As was the case with the prosthesis of FIGS. 1 to 7, the inlay30 in this embodiment is slidably displaceable on the flat and smoothupper surface 15 of seating member 14 between a forward/anteriorposition for a full extension of the joint, e.g. as shown in FIG. 28a ,and a rearward/posterior position for a full flexion of the joint, e.g.as shown in FIG. 29 a.

When the inlay 30 is in the forward or anterior position of FIG. 28a , abase 18′ of the projecting member 16 adjacent the surface 15 (which base18′ is formed as a flange in this example) abuts or contacts the curvedposterior or rearward end 39 of the aperture or slot 37, as was also thecase in the earlier embodiment. With reference to FIG. 28b , it will beseen that, just like the articulation surfaces 32, the curved uppersurface 23 of the projecting member 16 has a constant radius R′ in theantero-posterior or sagittal plane. The centre of curvature C′ of theupper surface 23, however, is offset in the posterior direction from thecentre of curvature C of the articulation surfaces 32 when the inlay 30is in the position shown in FIG. 28b . Thus, in this position, theprojecting member 16 extends upwards beyond the articulation surfaces 32of the inlay 30 into the central elongate opening 58 in ceramic body 50of the femoral component 3, ensuring lateral stability of the joint ismaintained.

As the femoral component 3 moves relative to the tibial component 2towards the fully flexed position of the joint shown in FIG. 29a , theinlay 30 is displaced in the posterior direction on the seating member14 until the anterior end 39 of the slot or aperture 37 in the inlay 30abuts a forward side of the base of projecting member 16. With referenceto FIG. 29b , it will be seen that the centre of curvature C′ of theupper surface 23 of the projecting member 16 in this positionsubstantially coincides with the centre of curvature C of thearticulation surfaces 32. In this position, therefore, the femoralcomponent 3 is able to slide over the upper surface 23 of the projectingmember 16, instead of the end 59 of the elongate opening 58 in theceramic body 50 abutting the projecting member 16. Thus, a greaterdegree of flexion is permitted in the joint and the movement does notterminate in a hard or rigid stop.

It will be noted with reference to FIG. 29b that the projecting member16 even in the fully-flexed position extends upwardly slightly beyondthe articulation surfaces 32; i.e. the radius R′ of the upper surface 23of the projecting member 16 is slightly less than the radius R of thearticulation surfaces 32. As will be apparent from FIG. 26, however, theprojecting member 16 is located in a central region 51 between thearticulation surfaces 56 of the femoral component. This region 51 formsa groove or depression that is slightly recessed with respect to thearticulation surfaces 56 such that it may move or slide over the uppersurface 23 of the projecting member 16 at this end of the jointmovement. The fact that the upper end of the projecting member isreceived within this slight recess 51 therefore continues to providelateral stability to the joint. As a result, this embodiment provides anenhanced range of movement in the joint as can be seen in FIG. 29a ,while nevertheless providing the patient with the desired stabilitythroughout that range movement.

With reference now to FIGS. 30 and 31 of the drawings, an embodiment ofa uni-compartmental prosthesis for a partial knee anthroplasty isillustrated. As will be appreciated from these schematic illustrations,this prosthesis is for implantation at one condyle (i.e. at one side) ofthe knee only, but nevertheless incorporates all of the core features ofthe present invention. For the purpose of simplicity, the anchor member12 of the tibial component 2 is not specifically illustrated in thesedrawings. It will be understood, however, that the fixation portion 10of the tibial component 2 may include one or more element depending fromthe seating member 14 to assist in anchoring the component in the bonetissue of the tibia. The seating member 14 includes an upstandingshoulder or rim 25 which may act as a stop or limit for movement of theinlay 30 in a transverse or lateral direction. As described above, thearticulation surfaces 32, 56 of the inlay 30 and the femoral componentbody 50 remain congruent over their full range of relative movement, andpreferably have a spherical profile of constant radius. As is also thecase with the other embodiments described above, the femoral component 3forms a cap or cover for an outermost region of the femur andessentially replaces the cartilage of the natural joint withoutrequiring removal of a substantial amount of bone tissue—i.e. merely toprovide stable seating surfaces and/or to receive a small positioningstud.

A particularly preferred embodiment of the uni-compartmental kneeprosthesis according to the present invention is illustrated in FIGS.32a-c , in which alternative stabilizing means for stabilizing theposition and movement of the bearing inlay 30 with respect to thefixation portion 10 of the tibial component 2 is illustrated. In thisembodiment, the stabilizing means includes a projecting member 42 whichprojects downwardly from the inlay 30 and is received in a slot orchannel 26 formed in the seating member 14. In particular, the slot orrecess 26 is formed in an upper surface 15 of the seating member 14, andthe sides and ends of the slot or channel 26 are configured to interactwith the member 42 projecting downwardly from the inlay 30 to definelimits to the translational and/or rotational movement of the inlay 30with respect to the fixation portion 10. In particular, the slot definesa path of translational travel for the inlay 30 over the upper surface15 of the seating member 14 between a most anterior position at one endof the slot 26 and a most posterior position at the opposite end.Similarly, as the inlay 30 rotates about a vertical axis, sides orflanks of the projecting member 42 are configured to engage the sideedge regions of the slot or channel 26 to limit further rotation, asclearly shown in FIG. 32b and in the cross-section of FIG. 32c (i.e.taken in the direction of arrows “32 c” in FIG. 32b ).

FIGS. 33a and 33b of the drawings respectively illustrate the mostposterior and most anterior positions of the bearing inlay 30 atopposite ends of the slot 26 along a path of translational travel overthe upper surface 15 of the seating member 14. FIGS. 34a to 34d of thedrawings illustrate possible variations in the configuration of theseating member 14 in the fixation portion 10 of the tibial component 2.In the embodiment of FIG. 34a , for example, the seating member 14 has amedial upstanding shoulder or rim 25 but no slot or channel 26. In theembodiment of FIG. 34b , the slot or channel 26 is provided in theseating member 14 to extend substantially parallel to the sagittalplane. In the embodiment of FIG. 34c , by contrast, the slot or channel26 is formed in the seating member 14 extending at an acute angle to thesagittal plane (e.g. in the range of about 5° to 20°). In the embodimentof FIG. 34d , on the other hand, the slot or channel 26 forms a curvedpath between the most posterior and most anterior positions of thebearing inlay 30.

With reference now to FIGS. 35 to 37 of the drawings, an apparatus 100for producing a knee joint prosthesis according to the invention, and inparticular for finishing the articulation surfaces of that prosthesis,is illustrated. The apparatus 100 of this embodiment has a frame 60comprising a base plate 61 and a structure of elongate frame members 62for supporting operational parts of the apparatus. To produce a kneejoint prosthesis 1 according to the invention, a ceramic bearing inlay30 for the tibial component 2 of the prosthesis is provided, the inlay30 having at least one articulation surface 32. In addition, a ceramicbody 50 for a femoral component 2 of the prosthesis is also provided,with the ceramic body 50 presenting at least one articulation surface 56for engagement with the respective articulation surface 32 of the inlay30. Importantly, in providing these two ceramic parts with one or morearticulation surfaces 32, 56 for engagement with one another, thearticulation surfaces are formed to have substantially complementary orconforming surface profiles. These substantially complementary orconforming surface profiles are initially generated either when formingof the ceramic parts themselves, e.g. by sintering within a negativeform or mould, and/or in a preliminary machining operation.

In order to finish the articulation surfaces 32, 56 of these ceramicparts 30, 50 of the tibial and femoral components 2, 3 and to renderthese surfaces adequately smooth and congruent for the knee jointprosthesis 1, the method and apparatus 100 of the invention provide forlapping of the ceramic body 50 with the ceramic inlay 30 in the presenceof a working substance S, such as a diamond polishing liquid or paste.In this connection, the apparatus 100 includes a receptacle 65, whichforms a tank or bath for holding a volume of the polishing liquid S. Theapparatus 100 further includes a first holding unit 70, such as aclamping device or chuck, for mounting and securely holding the bearinginlay 30—in this case, in what is essentially an in-use orientation suchthat the lower surface 35 is substantially horizontal and with the pairof articulating surfaces 32 facing upwardly. The first holding unit 70for mounting the bearing inlay 30 in this manner is arranged within thereceptacle or tank 65 for the polishing fluid S. This receptacle 65 andthe first holding unit 70 are preferably arranged so as to be adjustablypositionable in two dimensions over the area of the base plate 61.

The apparatus 100 further includes a second holding unit 80, such as aclamping device or chuck, for mounting and securely holding the ceramicbody 50 of the femoral component 3. The second holding unit 80 isparticularly designed to hold the ceramic body 50 for rotation of theceramic body 50 about an axis X which extends through the centres ofcurvatures C of the pair of articulating surfaces 56. Furthermore, inthis particular embodiment, the second holding unit 80 is arranged on amechanism 90 for adjustable movement of the second holding unit 80 atleast in a vertical direction, via a hydraulic or pneumatic ram 91.Desirably, however, the mechanism 90 is optionally also adapted foradjustable movement of the second holding unit 80 in a horizontal planerelative to the first holding unit 70 and the bearing inlay 30 heldtherein.

In operation, once the ceramic bearing inlay 30 has been securelymounted in the first holding unit 70 and the complementary ceramic body50 has been securely mounted in the second holding unit 80, the relativepositions of the first and second holding units 70, 80 are adjusted(e.g. via the mechanism 90) so that the respective articulation surfaces32, 56 of the inlay 30 and the body 50 are brought into proper andprecise alignment with one another. The receptacle 65 may then be filledwith polishing liquid or paste S such that the articulating surfaces 32of the ceramic inlay 30 are covered by the fluid. The ceramic body 50held in the second holding unit 80 may then be carefully lowered withthe ram 91 into the receptacle 65 so that the ceramic body 50 iscombined with the ceramic inlay 30 such that their respectivearticulation surfaces 32, 56 engage one another, with the polishingfluid located there between.

The apparatus 100 further comprises means for rotating the secondmounting unit 80 and the ceramic body 50 held therein relative to theceramic inlay 30 about the rotational axis X. In this embodiment, therotating means includes a drive 95 which is connected to a shaft 85 viaa transmission arrangement 96 and imparts reciprocal rotational movementto the ceramic body 50 held in the second mounting unit 80 relative tothe ceramic bearing inlay 30 held fixed in the first holding unit 70.

In addition, the apparatus 100 is designed to apply a predetermined orregulated force F in a vertical direction to the ceramic inlay 30 andceramic body 50 to enhance the finishing of the articulation surfaces.In this regard, the force F is imparted as a contact pressure betweenthe engaging articulation surfaces 32, 56 and can be set and/or adjusteddepending on the type of working fluid and the amount of materialremoval desired. For example, when using an abrasive working fluidhaving a relatively coarse particle size, with which a relatively highmaterial removal rate of about 100 μm/min can be obtained, a contactpressure in the range of 30 N/cm² to 40 N/cm² is employed. On the otherhand, when using an abrasive working fluid having a relatively fineparticle size with which a greater surface smoothness is to be achieved,contact pressure in the range of 3 N/cm² to 16 N/cm² is employed. Inthat case, a lower rate of material removal of 1 μm/min to 2 μm/min istypical.

As can be seen in FIG. 37, the force F is applied substantiallyvertically, e.g. via a static mass M and/or by the hydraulic ram 91, tothe mating or engaging articulation surfaces 32, 56. Rotation of theceramic body 50 of the femoral component 3 relative to the bearing inlay30 of the tibial component 2 about the axis X joining the centres ofcurvature C of the articulation surfaces thus effects polishing andfinishing of the articulation surfaces over the full range of movementof the components 2, 3 and across the full extent of the articulationsurfaces.

It will be appreciated that the above description of the preferredembodiments of the invention with reference to the drawings has beenmade by way of example only. Thus, a person skilled in the art willappreciate that various changes, modifications and/or additions may bemade to the parts particularly described and illustrated withoutdeparting from the scope of the invention as defined in the appendedclaims.

For example, where ceramic articulation surfaces are specified, a personskilled in the art will recognize and appreciate that such ceramicsurfaces typically comprise surfaces of a monolithic ceramic structure.However, a skilled person will also appreciate that such surfaces mayalternatively comprise a ceramic portion or layer carried by anon-ceramic substrate, such as a composite structure in the form of ametallic substrate having a ceramic portion thereon. Thus, no limitationon the invention is intended by way of the foregoing description andaccompanying drawings, except as set forth in the appended claims.

Furthermore, the skilled person will appreciate that ceramic materialssuitable for use in the ceramic inlay 30 and the ceramic body 50 includealuminium oxide ceramics, such as Al 999 (>99.8% Al₂O₃), zirconium oxideceramics, such as ZrO₂-TZP, ZrO₂-TZP-A, and ZrO₂-ATZ, and mixedceramics, such as zirconium-reinforced aluminium oxide having e.g. 81%Al₂O₃ and 17% ZrO₂. These ceramics are commercially available undertrade names such as Biolox® forte or Biolox® delta from CeramTec or BioHIP® from Metoxit.

In addition, it will be noted that the general dimensions of the kneejoint prosthesis 1 according to the invention are consistent with thedimensions of conventional knee joint prostheses known in the art. Inthis regard, it will be appreciated that the size of the tibial andfemoral components is largely dictated by the size of the natural kneejoint in a patient. To this end, it is typical to provide the prosthesesin a range of sizes or dimensions to suit different patients.

We claim:
 1. A knee joint prosthesis comprising: a tibial componenthaving a fixation portion comprised of a ceramic material adapted to befixed to an upper end of a prepared tibia in a patient, and a bearingportion presenting at least one articulation surface formed from aceramic material; and a femoral component adapted to be fixed to a lowerend of a prepared femur in a patient, the femoral component having abody portion presenting at least one articulation surface formed from aceramic material for engagement with the respective articulation surfaceof the tibial component, wherein the articulation surfaces of the tibialand femoral components are configured for essentially congruentengagement over a full range of movement of the prosthesis, thearticulation surfaces following a curve in a sagittal plane defined by aconstant radius over a full extent of their engagement, wherein thebearing portion of the tibial component is adapted for translational androtational movement relative to the fixation portion during said fullrange of movement of the prosthesis, wherein an axis of rotationalmovement of the bearing portion relative to the fixation portion isconfigured for movement with the bearing portion, wherein a projectingmember extending from the fixation portion is received or accommodatedin an aperture in the bearing portion and extends beyond the at leastone articulation surface of the bearing portion, and wherein theprojecting member includes one or more abutment member to limit thetranslational and rotational movement of the bearing portion relative tothe fixation portion.
 2. The knee joint prosthesis according to claim 1,wherein each of the articulation surfaces consists essentially of apartial circular-cylindrically shaped surface, or a partially sphericalsurface, or a partial toric-shaped surface.
 3. The knee joint prosthesisaccording to claim 1, or 2, wherein the bearing portion of the tibialcomponent presents two laterally spaced apart articulation surfaces, andthe body portion of the femoral component presents two correspondinglaterally spaced apart articulation surfaces for engagement with therespective articulation surfaces of the tibial component.
 4. The kneejoint prosthesis according to claim 3, wherein each of the articulationsurfaces of each of the tibial and femoral components has a separatecentre of curvature, and wherein the respective centres of curvature arespaced from one another in the medio-lateral direction.
 5. The kneejoint prosthesis according to claim 1, wherein the bearing portion isadapted for translational movement relative to the fixation portion inan antero-posterior direction, and wherein the bearing portion isadapted for rotational movement relative to the fixation portion about asubstantially vertical axis.
 6. A knee joint prosthesis comprising: atibial component comprising a fixation portion formed of a ceramicmaterial and adapted to be attached to an upper end of a prepared tibiain a patient, and a bearing portion formed from a ceramic material andpresenting at least one articulation surface, wherein the at least onearticulation surface follows a curve in a sagittal plane defined by aconstant radius over a full extent of said articulation surface, whereinthe bearing portion is adapted for translational and rotational movementrelative to the upper end of the tibia during a full range of movementof the prosthesis implanted in the patient, wherein an axis of therotational movement of the bearing portion is movable relative to theupper end of the tibia, wherein a projecting member extending from thefixation portion is received or accommodated in an aperture in thebearing portion and extends beyond the at least one articulation surfaceof the bearing portion, and wherein the projecting member includes oneor more abutment member to limit the translational and rotationalmovement of the bearing portion relative to the fixation portion.
 7. Theknee joint prosthesis according to claim 6, wherein the bearing portionof the tibial component presents two laterally spaced apart articulationsurfaces and wherein each of the articulation surfaces has a separatecentre of curvature.
 8. The knee joint prosthesis according to claim 6or 7, further comprising: a femoral component adapted to be fixed to alower end of a prepared femur in a patient, the femoral component havinga body portion formed from a ceramic material and presenting at leastone articulation surface for engagement with the respective articulationsurface(s) of the tibial component, wherein the articulation surfaces ofthe tibial component and the femoral component are configured tomaintain essentially congruent engagement over a full range of movementof the prosthesis.
 9. The knee joint prosthesis according to claim 1 or6, wherein the fixation portion of the tibial component comprises aseating member upon which the bearing portion is movably supported, thebearing portion being adapted for sliding movement on the seatingmember, and wherein the seating member is configured as a plate-likemember, an upper surface of which forms a seating surface upon which thebearing portion is supported and is adapted to move relative to thefixation portion.
 10. The knee joint prosthesis according to claim 1 or6, wherein the fixation portion of the tibial component comprises ananchoring member to be inserted into a recess formed in an upper end ofthe prepared tibia, the anchoring member being adapted to extend intothe tibia and being configured to inhibit rotation of the fixationportion relative to the tibia.
 11. The knee joint prosthesis accordingto claim 1 or 8, comprising joint stabilising means adapted to limit therelative movement between the tibial component and the femoralcomponent, wherein the joint stabilising means comprises a member whichdefines a limit to the movement of the prosthesis at a fully extendedposition and/or at a fully flexed position, and wherein the stabilisingmeans is adapted to retain the articulation surfaces of the tibial andfemoral components in registration with one another to prevent thecomponents from inadvertently moving or slipping, in a lateraldirection, out of proper engagement or alignment.
 12. The knee jointprosthesis according to claim 11, wherein the bearing portion of thetibial component presents two laterally spaced apart articulationsurfaces, and the body portion of the femoral component presents twocorresponding laterally spaced apart articulation surfaces forengagement with the respective articulation surfaces of the tibialcomponent, and wherein the joint stabilising means comprises aprojecting member located upstanding between, and extending upwardlybeyond the two laterally spaced articulation surfaces of the tibialcomponent, whereby the projecting member is within an opening oraperture formed between the corresponding articulation surfaces of thefemoral component of the knee prosthesis.
 13. The knee joint prosthesisaccording to claim 12, wherein the projecting member of the tibialcomponent cooperates with the opening or aperture of the femoralcomponent to form means for registering the position of the femoralcomponent on the tibial component and means for laterally stabilizing aposition of the femoral component relative to the tibial component, andwherein the projecting member extends from the fixation portion throughan aperture in the bearing portion to form a pivot for rotationalmovement of the bearing portion relative to the fixation portion of thetibial component.
 14. The knee joint prosthesis according to claim 11,wherein the member of the joint stabilising means which defines a limitto the movement of the prosthesis at a fully extended position and/or ata fully flexed position comprises an end region of a channel formed inthe femoral component, wherein the end region of the channel is roundedor curved for substantial conformity with a corresponding side of aprojecting member which extends upwards from the fixation portionthrough an aperture in the bearing portion to be received within thechannel formed in the femoral component.
 15. The knee joint prosthesisaccording to claim 1 or 6, wherein the one or more abutment member formsa stop against further relative movement of the bearing portion.
 16. Theknee joint prosthesis according to claim 15, wherein the one or moreabutment member is adapted to engage the bearing portion within theaperture in the bearing portion for receiving the projecting member tolimit the translational and rotational movement thereof relative to thefixation portion.
 17. The knee joint prosthesis according to claim 16,wherein the one or more abutment member is provided on the fixationportion in the form of an extension or protrusion from the projectingmember.
 18. The knee joint prosthesis according to claim 15, wherein theone or more abutment member is configured or positioned to delimit apredetermined maximum angle of rotation of the bearing portion relativeto the fixation portion.
 19. The knee joint prosthesis according toclaim 15, wherein the one or more abutment member is configured orpositioned to delimit a predetermined maximum displacement and/or apredetermined maximum angle of rotation of the bearing portion relativeto the fixation portion.
 20. The knee joint prosthesis according toclaim 15 wherein the one or more abutment member has a geometryconfigured for surface contact with the bearing portion.
 21. The kneejoint prosthesis according to claim 1 or 6, wherein the means forlimiting movement includes one or more side or end of a slot or channelformed in the fixation portion of the tibial component and configured tointeract with a member projecting downwardly from the bearing portion todefine limits to the translational and rotational movement of thebearing portion.
 22. The knee joint prosthesis according to claim 1 or6, wherein the bearing portion is placed downwards onto the fixationportion axially along a vertical axis of the fixation portion.
 23. Theknee joint prosthesis according to claim 1 or 6, wherein the prosthesisis a uni-compartmental prosthesis for partial knee arthroplasty.
 24. Theknee joint prosthesis according to claim 1, wherein the bearing portionis configured to be assembled with the fixation portion by placementthereof downwards from above onto the fixation portion.